In coherent optical communication, a frequency offset (a frequency error), which is a frequency difference, occurs between a frequency of a received signal and a frequency of a local oscillation light source. Further, due to a nonlinear optical effect, vibration of an optical fiber and the like, a phase fluctuation such as phase noise occurs in the received signal.
To cope with this, a technique of, after raising an input signal to the power of N to remove a phase term (a phase change due to modulation), detecting a frequency error and feeding back the frequency error to an input side to compensate a frequency offset is proposed (see, for example, PTL 1). However, though the frequency offset can be compensated to some extent, a lot of phase noise still remains.
Further, a technique of tentatively determining a received signal based on a threshold set according to an amplitude and compensating a difference between an original phase and a phase of the received signal is proposed (see, for example PTL 2). However, since accuracy of phase noise compensation is low only by tentative determination, the accuracy is increased by feeding back a frequency error and a phase error detected during calculation to reduce a phase fluctuation before the tentative determination.
Further, a technique of extracting a known pattern inserted on a transmission side from a received signal and detecting a difference between an original phase and a phase of the received signal to compensate phase noise is proposed (see, for example, PTL 3).